Not all CAD formats are created equal. In fact just because they have the same suffix, .stp or .igs or any other format type doesn't guarantee that they will behave in CADstyle or other even CAD programs on import. There is no bullet proof standard.

Whenever I've had issues with imports, it's been export settings that had too much detail in the fillets/radius edges. You might try tweaking the settings and try again. Additionally, if the CAD app supports some other export formats, you might try work with one  of those as an alternative.

Also, make sure that you are using Carrara v6.2.1 - there were some improvements.

Mark